"Sex", probably. And not for the reason you'd think.
For me, personally, it invokes a sort of "oh christ, what is this shit" reaction. "Free" is just everywhere, so it's mostly ignored.
Think of the amount of times you have skimmed over something that said "FREE" in big, neon capital letters. It doesn't even register.
And now think of "SEX" used the same way. You will stop - perhaps even scroll up, if necessary - just to see what the fuck it is on about.
I guess some people might just be like "hurrdurr teh sax? lol k i want sum". I would like to think most of the time it is for my reason, though.
EDIT: Oh, what would make me more likely to click on a link. Right. Whoops.
All wake and no sleep so on so forth.
Neither. There is no way either of those links will be anything but dodgy.